1. Field
The present invention relates generally to communications, and more specifically to estimating the signal to interference-plus-noise ratio (SINR) of a wireless channel.
2. Background
A modern day communication system is required to support a variety of applications. One such communication system is a code division multiple access (CDMA) system which conforms to the “TIA/EIA/IS-95 Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System”, hereinafter referred to as the IS-95 standard. The CDMA system allows for voice and data communications between users over a terrestrial link. The use of CDMA techniques in a multiple access communication system is disclosed in U.S. Pat. No. 4,901,307, entitled “SPREAD SPECTRUM MULTIPLE ACCESS COMMUNICATION SYSTEM USING SATELLITE OR TERRESTRIAL REPEATERS”, and U.S. Pat. No. 5,103,459, entitled “SYSTEM AND METHOD FOR GENERATING WAVEFORMS IN A CDMA CELLULAR TELEPHONE SYSTEM”, both assigned to the assignee of the present invention and incorporated by reference herein. The “TIA/EIA/IS-2000 Standard” describes a next generation cdma2000 multi-carrier 1X and 3X air interface specification, hereinafter referred to as the cdma2000 standard.
In the CDMA system, communications between users are conducted through one or more base stations. In this specification, a base station refers to the hardware with which user terminals communicate. A first user terminal communicates with a second user terminal by transmitting data on a reverse link to a base station. The base station receives the data and can route the data to another base station. The data is transmitted on a forward link of the same base station, or a second base station, to the second mobile station. The forward link refers to transmission from the base station to a user terminal and the reverse link refers to transmission from the user terminal to a base station. In IS-95 systems, the forward link and the reverse link are allocated separate frequencies.
Given the growing demand for wireless data applications, the need for very efficient wireless data communication systems has become increasingly significant. The IS-95 standard is capable of transmitting traffic data and voice data over the forward and reverse links. A method for transmitting traffic data in code channel frames of fixed size is described in detail in U.S. Pat. No. 5,504,773, entitled “METHOD AND APPARATUS FOR THE FORMATTING OF DATA FOR TRANSMISSION”, assigned to the assignee of the present invention and incorporated by reference herein. Further, a high data rate (HDR) system that provides for high rate packet data transmission in a CDMA system is described in detail in the “TIA/EIA/IS-856—cdma2000 High Rate Packet Data Air Interface Specification” (hereinafter referred to as the HDR standard), as well as U.S. Pat. No. 6,574,211, entitled “METHOD AND APPARATUS FOR HIGH RATE PACKET DATA TRANSMISSION”, issued Jun. 3, 2003, assigned to the assignee of the present invention and incorporated by reference herein.
According to these HDR systems, user terminals transmit a data rate control (DRC) message to the base station. The DRC value corresponds to the rate at which the user terminal expects to receive data on the forward link. The DRC value depends, at least in part, on the signal to interference-plus-noise ratio (SINR) of the user terminal's channel. Less noisy channels can support higher data rates. The DRC value should therefore be decreased as the channel SINR decreases, or increased as the channel SINR increases. DRC values should be set as high as possible if data throughput in the system is to be maximized. However, accurately setting the DRC value depends upon achieving a reliable estimate of the channel SINR. Overestimating the channel SINR can result in a data rate that results in an unexpectedly high occurrence of errors at the user terminal.
An adaptive equalizer can be used in the user terminal's receiver to suppress interference and improve system performance. The filter coefficients of the adaptive equalizer can be adapted (referred to herein simply as adapting the adaptive equalizer) during the pilot portion of a particular frame. As used herein, describing the adaptive equalizer as being adapted or applied “during” a particular portion of a frame refers to adapting or applying the equalizer to the symbols within that portion of the frame, though the symbols need not necessarily be processed during any particular time interval. Frames (also referred to herein as packets) transmitted in a HDR system include the pilot portion as well as one or more non-pilot portions, such as a control portion and a data portion. The pilot symbols are known a priori at the receiver. Conventional algorithms for adapting the filter coefficients to their desired values are often based on the criteria of minimizing mean square error (MMSE) between the known pilot symbols and the equalizer's estimates of these pilot symbols. Two common examples of adaptive MMSE algorithms are the least-mean-square (LMS) algorithm and the recursive-least-squares (RLS) algorithm.
Typically the known pilot symbols make up a fraction (e.g., 10%) of a frame, while control symbols and data symbols make up the remainder of the frame (e.g., 10% and 80%, respectively). Though the pilot symbols are known at the receiver, the control symbols and data symbols are a priori unknown at the receiver. In many situations (such as when the uncoded symbol error rate is high enough to prevent decision directed adaptation, or when low implementation complexity is desired) the equalizer coefficients are recursively adapted only during the pilot portion of the frame and held fixed during the non-pilot portions of the frame.
According to conventional receiver designs, the channel SINR is calculated using one or more parameters estimated during the pilot portion of a particular frame. These estimated parameters could include the mean squared error (MSE) of the equalizer's estimates of the pilot symbols, the bias in the equalizer's estimates of the pilot symbols, or some other estimates. As will be apparent to those skilled in the art, the SINR can be defined in different ways depending on a variety of assumptions made regarding the application environment. However, according to many definitions, the SINR is inversely related to the MSE.
In those instances where the equalizer's coefficients are adapted during the pilot interval alone, the difference between the pilot symbols and the equalizer's estimates of the pilot symbols will on average be smaller than the difference between the data symbols (or control symbols) and the equalizer's estimates of the data symbols (or control symbols). In other words, the MSE of the equalizer will be lower during the pilot portion than it will be during non-pilot portions. This MSE difference arises because the adaptive filter coefficients are tuned by the adaptive algorithm to minimize the MSE during the pilot portion, but are not specifically tuned to the non-pilot portions. The magnitude of the difference will depend, in part, on the extent to which the pilot portion is not statistically representative of the underlying random process that describes the non-pilot portions. This phenomenon is referred to herein as the adaptive equalizer being “overfit” to the pilot portion. The overfit is magnified when algorithms such as the multiple-pass LMS are used because with each successive pass through the same observations, the coefficients tune themselves more closely to the particular data set.
Overfitting of the adaptive equalizer affects not only the calculation of MSE, but calculation of other parameters as well, such as the bias in the adaptive equalizer's estimate. As mentioned above, these factors can be used to estimate the channel SINR. Errors in the calculation of these parameters will result in errors in the estimate of the channel SINR. This in turn can cause the receiver to select a sub-optimum DRC value. For example, receivers that calculate the MSE during the pilot interval may overestimate the channel SINR due to the equalizer being overfit to the pilot symbols. If the DRC value is selected based on this SINR estimate, the transmitter may then transmit frames at a data rate that will result in a larger number of errors than expected in the received data. This is because the quality with which a frame is received (e.g. the packet error rate (PER)) depends, at least in part, on the data rate and the channel SINR experienced during the data portion of the frame.
There is therefore a need in the art for an improved method and apparatus for estimating the SINR of a wireless channel that compensates for overfitting of an adaptive equalizer in the receiver.